The field of genetics is quite young, scientifically speaking! Although Gregor Mendel is often called “the father of genetics,” he actually did not know about the existence of genes when he performed his now-famous inheritance experiments using pea plants in the mid-1800’s.
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The word “gene” was first used much later, in 1909, by Danish botanist Wilhelm Johannsen to represent the basic unit of inheritance. Many scientists thought that genes were formed into structures called chromosomes inside our cells, and this was confirmed in 1910 by Thomas Hunt Morgan. Famously, the discovery by Rosalind Franklin of the double-helix structure of DNA was published in 1953 by James Watson and Francis Crick. 

The first test to sequence DNA was performed on bacteria in 1972.